bpo-46307: Add string.Template.get_identifiers() method (GH-30493)

Add `string.Template.get_identifiers()` method that returns the identifiers within the template. By default, raises an error if it encounters an invalid identifier (like `substitute()`). The keyword-only argument `raise_on_invalid` can be set to `False` to ignore invalid identifiers (like `safe_substitute()`).

Automerge-Triggered-By: GH:warsaw